A large number of duct leak and overheat detection systems in operation today are zone-type systems. As with any duct leak and overheat detection system, sensor loops, comprising elongate sensing cables, are installed along the aircraft's bleed air duct and adjacent heat sensitive components. The sensors are configured in loops, and there are one or two loops for each zone being monitored. The problem with these systems is that during maintenance of the aircraft it is difficult to locate exactly where, along the length of the loop, the overheat event had occurred. In situations where faster maintenance action is warranted, the loops are broken down into smaller more manageable sections, but at the expense of more wires and associated weight. In cases where the loop is monitoring a very long section of ducting, such as that which might be associated with the auxiliary power unit in certain aircraft, troubleshooting a duct leak can take days to accomplish.
Systems have been configured for newer aircraft which incorporate a maintenance feature that automatically detects and displays the location of the overheat event. These systems utilize a controller which measures current into both ends of the cable loop, and performs a bridge-type calculation to determine or estimate where in the loop a fault had occurred. These systems often use special sensor cables of fairly precisely controlled resistance per unit length, and the controllers are configured such that event location and overheat detection are performed by common hardware and software.
The newer controllers, however, are not suitable for installation in existing aircraft with existing cable systems. To install a newer system in an existing aircraft would typically require the removal of the existing duct leak and overheat detection system, including the cables and the controller, and the replacement with a completely new system.
Components associated with the in-flight operation of the aircraft are certified for use only after extensive testing according to strict protocols. Thus, for example, if one were to attempt to modify an existing overheat detection system controller to provide it with event location capabilities, and the addition required an actual modification to the overheat detection controller, the entire controller would need to be retested and re-certified. This is particularly troublesome for aircraft operators because the event location system is intended solely for maintenance personnel, although it interfaces with the overheat detection system which is operative during flight. The overheat detection system provides indicators in the cockpit for signaling the pilot of any overheat conditions, and also has test buttons which allow the pilot to test the system, for example, during pre-flight checks. It would be highly undesirable to require re-certification of the overheat detection system solely to provide a maintenance capability to assist the maintenance personnel.